A variety of optical devices make use of optical gratings such as Echelle gratings. In addition to the gratings, these devices typically include an input waveguide and a plurality of output waveguides. During operation of the device, a multi-channel light signal travels from the input waveguide to the grating and is reflected off the grating toward the output waveguides. The reflected light signal separates into output signals that each carries one of the channels. Each of the output waveguides is positioned to receive one of the output signals. Accordingly, each output waveguide is associated with a particular one of the output signals.
The reflection of the light signals off of a grating with non-ideal facet verticality excites higher order modes that disperse as they travel from the grating to the output waveguides. The dispersion of these higher order modes can cause a portion of the light from one of the output signals to be received at an output waveguide that is not associated with that output signal. Accordingly, the dispersion can be a source of cross talk and loss at the output waveguides. One method of dealing with this dispersion has been to improve the verticality of grating facet or make the device support only a single mode. However, the vertically of grating facet is limited by fabrication capability and single mode devices are often not desirable especially for Silicon on Insulator (SOI) waveguide.
Accordingly, there is a need for an optical device that includes an optical grating with reduced mode dispersion.